Okey, I kind of figured out it must be this flat flying thing round or rectangular, but what does PBF stand for? And anoter question where should the center of gravity be in a PBF ? And what size would you recommend for a .061 engine? If I would build one

PBF = Pizza Box Flyer. The name was based on the box the pizza comes in. But I've seen it used to refer to some other shapes as well as long as they aren't close to a flying saucer shape.

For hovering the balance point should be back very close to the aircraft neutral point. For a flying wing that point is generally agreed to be the 25% chord point. So on a 24 inch square PBF that would be 6 inches back. Most guys fly with it 4 inches back but check wtih the PBF threads in the 3D and funfly forums.

Folks have made 18 inch square PBF powered with 049's and 061's. I would imagine they tend to be a little nervous at that size though.